@Jonathan-Dupré
Two ways:
Long-press the crown -> Settings -> Display -> Always-on Display
Go to Widgets -> Control Panel -> Always-on Display
Note: During an activity, you must first exit the activity (not stop it). Long-pressing the bottom button takes you to the watch face. Once the setting is complete, pressing the crown returns you to the activity.
Unfortunately, this option isn’t available in the activity’s Sports Settings. It’s better to set it beforehand.